1. Field of Invention
This invention relates to the use of an evaporative foam risers made of expanded polystyrene with a cavity on the upper surface that is filled with exothermic topping which is used to increase the feeding efficiency of a conventional blind riser.
2. Description of Prior Art
Risers are devices that supplies metal to the casting while in the process of solidifying. Without an adequately sized riser, the casting will contain voids (shrink holes) that develop when the casting solidifies and its volume contracts. Risers are therefore essential in obtaining a sound casting.
Risers can be encased in sand or in an insulating or exothermic material. The use of insulating and exothermic sleeves is a common practice in making casting. Normally, the most common type of riser sleeve is manufactured from fiber based refractory mineral. The pores of the fibers are impregnated with exothermic material while under vacuum. These materials extend the solidification time of the riser. The longer the extension of the solidification time, the smaller is the size of the required riser, thus increasing the yield. The use of these sleeves, however, has some disadvantages:    They are considerably more expensive than the equivalent sand riser.    Because of their weight, they tend to fall off while transporting the mold thus causing scrap.    The refractory fibers do not burn off during the casting process thus it contaminates the sand system and may cause casting defects.
The use of sand (encased) riser is the most common application in the manufacture of castings. They usually are part of the pattern equipment. They are inexpensive to use but because they lack a source of additional heat, they have shorter solidification time than the exothermic sleeve riser and thus requires more metal.